[Haskell-cafe] Newbie on instance of Monad

[Henning Thielemann]

On Fri, 31 Oct 2008, Jonathan Cast wrote:

> On Fri, 2008-10-31 at 18:43 -0200, Mauricio wrote:
>> Hi,
>>
>> After a lot of thinking, I can't get what I
>> am doing wrong in this code:
>>
>> ------
>> data ( RandomGen g ) => RandomMonad g a = RandomMonad (g -> a)
>>
>> instance Monad (RandomMonad g) where
>>    return = RandomMonad . const
>>    RandomMonad f1 >>= f2 = RandomMonad f3 where
>>      f3 a = f2f1 a (next a)
>>      RandomMonad f2f1 = f2 . f1
>
> Yikes.  Just search the archives for `Set not a Monad'; you have the
> same issue.

I think that this would be the answer if he is after a constraint for 'a', 
but he wants to constraint 'g'.

> Hint: data RandomGen g => RandomMonad g a means nothing at all like what
> you think it means.